JP2009-123550A describes a conventional fuel cell system in which a part of cathode gas discharged from a cathode compressor is discharged to a cathode gas discharge passage by flowing the part of cathode gas into a bypass passage if the part of cathode gas is unnecessary for power generation of a fuel cell stack. In this way, this conventional fuel cell system controls a flow rate of the cathode gas supplied to the fuel cell stack (stack supply flow rate) to a target stack supply flow rate set according to a power generation request.